Integrated circuit technology has revolutionized various fields, including computers, control systems, telecommunications, and imaging. In the field of imaging, complimentary metal oxide semiconductor (CMOS) active pixel image sensors have made considerable inroads into applications served by charge coupled imaging devices. As noted in U.S. Pat. No. 5,625,210 to Lee et al. (“the '210 patent”), an active pixel sensor refers to an electronic image sensor with active devices, such as transistors, that are associated with each pixel. The active pixel sensor has the advantage of being able to incorporate both signal processing and sensing circuitry within the same integrated circuit because of the CMOS manufacturing techniques.
One common design for an active pixel is the basic, three-transistor CMOS active pixel which contains a photodiode; a reset transistor for resetting the photodiode voltage, a source follower for amplification, and a row select transistor for buffering the photodiode voltage onto a vertical-column bus. However, the three-transistor pixel is lacking in its ability to suppress noise due to the reset operation, referred to as kTC noise. Further, the three-transistor pixel does not have good response to blue light.
Another popular active pixel sensor structure consists of four transistors and a pinned photodiode. The pinned photodiode has gained favor for its ability to have good color response for blue light, as well as advantages in dark current density and image lag. Reduction in dark current is accomplished by “pinning” the diode surface potential to the Pwell or Psubstrate (GND) through a P+ region. Because of the particular characteristics of pinned photodiodes, it is necessary to incorporate a transfer transistor that is not required in the three-transistor design discussed above.
Still, one disadvantage of the pinned photodiode is that it requires four transistors for each pixel. Thus, a one-megapixel image sensor would require 4 million transistors simply for the imaging array. As higher resolution image sensors become popular, coupled with the need for higher integration densities, it is desirable to implement the pinned photodiode pixel while limiting the number of required transistors.